Ambient condition detection systems, such as fire alarm systems, are often incorporated into commercial and industrial buildings for the protection of people and property in the respective buildings in the event of a fire. Such systems often incorporate one or more ambient condition detectors which are capable of detecting the presence of a selected condition, such as smoke, temperature, flame or toxic gases. The presence of the selected condition or conditions is often indicative of the presence of a potential fire. In such an instance, it is important to be able to provide audible and/or visual indications of an alarm condition in the region being supervised.
Devices which provide the discernable alarm indicating outputs such as strobe lights, sirens or the like are usually maintained in an unenergized state until an alarm condition has been detected. Once an alarm condition has been detected, the units are energized, usually by reversing the plurality of the associated energy supply, and thereupon start to function in their normal fashion.
Since the alarm indicating output units often need to be located at sites remote from a control unit which incorporates the driving power supplies or energy sources, electrical cables are used to interconnect the output devices with the control unit. The electrical cables at times are required to run substantial distances depending on the size of the region being supervised. In addition, there may well be a need for a relatively large number of output devices to insure adequate coverage over large regions.
Installation problems in such systems are compounded by the fact that the output devices often have an input impedance with a highly capacitive component. As such, when the electrical plurality to the devices is reversed, there can be a substantial transient in-rush current at each device. This in-rush current can far exceed the normal steady state current requirements for the device. The initial in-rush current imposes additional current requirements on the power supplies and also on the associated transmission cables.
Prior solutions to the in-rush current problem have not been as cost effective as desired. These have included incorporating higher capacity, more expensive power supplies into the control unit for the purpose of driving the output devices. Other attempted solutions have limited the number of devices on a given set of transmission cables. Alternately, the current carrying capacity of the transmission lines has been increased.
Hence, there still continues to be a need for cost-effective solutions to the in-rush current problem. Preferably, such solutions would enable installers to connect larger numbers of output devices to a common transmission cable without the need for increasing power supply size or cable size just to deal with the in-rush current problem. Preferably, such solutions could be incorporated into output devices which would exhibit lower initial in-rush currents and which could be used to retrofit existing systems thereby providing improved performance thereto.